Western officials are struggling to assess whether irreparable damage has been done to an already strained partnership with the Afghan military and government.

Reporting from Kabul, Afghanistan — The desert's nighttime chill had taken hold at a small U.S.-Afghan base in the Taliban's heartland: the home village, in fact, of Mullah Mohammed Omar, the movement's founder and supreme commander. For the American troops manning the outpost, though, the danger came not from outside the wire, but from within.

Hours before dawn Thursday, Afghan assailants, including a man hired to teach Afghan soldiers to read, shot and killed two U.S. troops and wounded a third, Afghan and American officials said. The soldiers slain at the base in Kandahar province were the fifth and sixth U.S. military personnel to die in a span of eight days at the hands of Afghans they had worked alongside.

With these latest killings, the proportion of NATO overall military fatalities caused by such "insider" shootings this year stood at nearly one in five.

The deaths come against a backdrop of deepening mutual mistrust between many Afghans and their Western counterparts after riots tore through the country last week over what officials said was the inadvertent burning of copies of the Koran at a U.S.-run military base.

In the wake of the violence, which has left more than three dozen dead, hundreds of Western military and civilian advisors working at Afghan government ministries were withdrawn by embassies and the NATO force. Troops at jointly run Afghan-coalition bases were ordered to keep their distance, and hold their tempers. Many foreign aid and development groups moved to isolate international staffs, citing safety fears. In Kabul, the capital, most Westerners took care to keep a low profile.

Although mass protests over the Koran burning died down at the end of last week, Western diplomats and military officials are still struggling to assess whether irreparable damage has been done to an already strained partnership with the Afghan military and government. That cooperation, fostered through years of painstaking efforts, lies at the heart of the North Atlantic Treaty Organization's hope of largely stepping back from its combat role by the end of next year.

It's a big-picture strategic question as well as a wrenchingly personal one. One Western civilian who has been working for months as an advisor to the Afghan government described a close Afghan colleague as being unwilling to meet her eyes after news of the Koran burning broke.

"It was a very, very painful moment," she said. "For me, and I think for them."

Some Afghans, for their part, said they considered the international pullback from government ministries a demoralizing blow, although a trickle of foreign advisors — mainly those with "mission-critical" jobs — began returning to work this week. Sayed Hameed Sadaat, who works with foreign advisors at the Labor Ministry, said their abrupt withdrawal gave the impression of a "weak commitment" on the part of the international community to Afghanistan.

Publicly, U.S. officials have painted the Koran incident as a setback, but scarcely one that could shatter longtime bonds. They point out that the rioters made up only a tiny fraction of the Afghan population, and assert that it was a situation in which the Taliban and other Islamist militants seized an opportunity to both whip up and blend into the crowds. The American ambassador, Ryan Crocker, told the BBC this week that he sees no "permanent rupture" arising from the episode.

But the Koran violence coincided, in perfect-storm fashion, with what has become a quickening drumbeat of so-called green-on-blue attacks: those carried out against foreign forces by Afghan allies. Even before the spate of American deaths that began Feb. 23, the year had gotten off to a bad start: Four French troops were shot and killed in January by an Afghan soldier acting in apparent sympathy with the insurgents, and an Albanian soldier was killed by gunmen in Afghan police uniforms Feb. 20 in Kandahar.

The six American fatalities since the Koran burning became public Feb. 21 include two U.S. troops shot Feb. 23 by an Afghan soldier at their base in eastern Afghanistan and two ranking officers — a major and a lieutenant colonel — shot point-blank at their desks Saturday in the command-and-control center in the Interior Ministry, one of the most tightly guarded Afghan government installations. A ministry worker is being sought in those deaths.

The NATO force said in a statement that Thursday's fatal shootings were carried out by a man believed to be an Afghan soldier, apparently acting in concert with a man in civilian clothing. Afghan officials suggested that the civilian, a literacy tutor working for the Afghan army, had managed to grab the weapon of an Afghan soldier.

NATO's International Security Assistance Force gave the location only as southern Afghanistan. Afghan officials said the base was in the village of Sangisar, in Kandahar province, where Mullah Omar made a name for himself as a village preacher in his pre-Taliban days in the early 1990s.